Enhancement of magnetoresistance in Mn substituted La2/3Sr1/3 Mn1-x Zr x O3 granular system

Granular metal/insulator (La2/3Sr1/3MnO3/Zr-contained compounds) composites were prepared by doping Zr in La2/3Sr1/3MnO3. The effects of Zr doping on structure and grain size, as well as magnetic and electric properties have been investigated. The result shows that only a very limited amount of Zr (x<0.05) can be doped into the lattice of La2/3Sr1/3MnO3 and most Zr ions segregate as the secondary compounds (SrZrO3, La2Zr2O7 and ZrO2). The grains in the undoped sample are relatively homogeneous with single average size of about 3 μm. For the low Zr-content samples (x=0.05 and 0.1), however, two distinct particles of large and small sizes coexist, and these large (or small) particles have a well-defined average size of their own. For x≥0.3, not only the grains become homogeneous again, but also their average size is significantly smaller than that of the undoped sample. It has also been found that with x increasing the paramagnetic insulator-ferromagnetic metal transition temperature T C (or T P) first decreases gradually (x≤0.3) and then stays nearly constant (x>0.3), while the resistance and low- and room-temperature magnetoresistance (MR) increase substantially. Interestingly, a large and nearly temperature-independent MR has been found in a broad temperature range (200-300 K) for the samples with x>0. 3